In the average movie theater, two types of "surround" systems are used--the 70 mm 6-track magnetic system, and the more common 35mm optical arrangement. The former uses a magnetic strip attached to the film to supply six discrete channels, and the latter uses two optical audio tracks. This two-channel system is the basis for home surround sound decoders.
Every stereo videodisc, tape and MTS broadcast that was surround encoded still contains the same rear channel information as the two-channel magnetic master from which the theatrical 35mm optical soundtrack was produced. In other words, your stereo videotape or disc of Star Trek I, II, III, Raiders of the Lost Ark, Superman and Star Wars can be decoded to produce surround sound at home. In addition, LPs, CDs and any stereo audio material can benefit from surround sound decoding. Ambiance extraction is a pleasant side effect that many decoders provide. In a nutshell, if the recording was made in a large hall, or a small club, "surround sound" will reproduce the recording environment faithfully.
Assuming the listener is seated centered between the two speakers, sound which is recorded "in phase" and with equal amplitude in each channel in a standard stereo system will appear to the listener to be located equidistant between the two speakers, as the two in-phase audio signals add together. The sound can be shifted left-to-fight by varying the ratio of the amplitude of the left and right signals.
By subtracting the left and fight signals (L-R), the in-phase signals will be cancelled, and the out-of-phase signals are recovered. This is the basis of the "matrix encoding" which is used to record surround information which is inaudible to listeners with conventional stereo equipment.
If a signal is recorded at equal amplitude on each channel of the stereo but 180.degree. out of phase, an exactly centered listener would ideally hear nothing, as the two signals cancel each other out. As a practical matter, the signals are audible, but sound odd--there is almost no centered sound at all, but the source appears to shift past the left or right speaker to be "out there somewhere". This accounts for the occasional feeling when playing a surround-encoded movie on conventional two-speaker stereo equipment that the sound is coming from behind or off to one side or the other.
In a conventional surround system there are four or five speakers. The standard left and right speakers of the normal stereo sound are located in front of the viewer to the left and right of the screen. The left and right rear (surround) signals are reproduced on speakers behind the listener. Many systems will also add a center (Left plus Right, or the same as monophonic) signal in front, under the screen, to add "centering" for dialog.
In recent years there have been two conflicting trends in more-expensive television equipment. First, it has become increasingly common to include stereo decoders in televisions, which in turn requires two speakers for sound reproduction. Second, the portion of the front of the TV which is occupied by the screen has become larger, until there is no room left for even one speaker, let alone two. As a result, although mono TV sets may still have a front-facing speaker (albeit small), the speakers in most stereo TVs have been moved to the sides of the cabinet, usually pointing outward. These closely spaced speakers provide little stereo effect, at best. At the same time, it has become more common than ever to place the TV into an "entertainment center" or bookcase, which surrounds the TV set and blocks the speakers. This results in a loss of whatever stereo effect there might have been.
As a result, many consumers have abandoned the sound system in their TV sets completely, using an external amplifier and speakers (in most cases an existing stereo setup) driven by the stereo output from the TV (if present), or from a VCR to drive two front speakers. This adds considerable expense to the cost of the television, not to mention the need for space for the two additional front speakers. These speakers often cannot be placed too close to the TV because of magnetic effects, and the need to center the TV between them for proper stereo effect restricts the placement of all of the components in the room.
As "surround encoded" videotapes and television transmissions have become more common in recent years there has been an influx of surround decoders on the market. Typical of these are the various Dolby.RTM. systems, or the Dynaco model QD-1, which is a version of the decoder described in a 1970 Audio Magazine article by David Hailer for use with the then-emerging quadraphonic sound technology (which has since been abandoned). Hafler's U.S. Pat. No. 3,697,692 is essentially the same as the Dynaco QD-1.
These systems require, at a minimum, a stereo amplifier (QD-1) or two (all low-level systems such as Dolby, THX, or the system described in co-pending application Ser. No. 08/184,648) and four or five separate speakers, with associated wiring and cables. The expense can be quite high and setting up such a system is not trivial, especially for the average non-technical consumer.
Several patents have suggested the use of three speakers for stereo setups. For example, see Sorkin, U.S. Pat. No. 3,478,167, or Klayman, U.S. Pat. No. 4,819,269. Neither of these are concerned with surround sound applications, but simply show three-speaker stereo arrangements using difference signals as part of the array of speakers.
Remote wireless speakers for audio applications have been known for some time. For example, see Mlodzikowski, et. al., U.S. Pat. No. 4,899,388, which uses an infrared transmitter to drive two remote battery powered speakers for a conventional stereo system. For many years General Electric sold a wired-wireless (carrier current) monophonic remote speaker under the "Porta-Fi" trademark, and Radio Shack also sold a similar wireless stereo remote speaker system around 1990. Accessory-maker Recoton's Model W100 wireless stereo speaker system also transmitted through the house AC wiring, and their newest wireless speakers use a 900 MHz radio transmitter/receiver setup. Headphone manufacturer Koss has a pair of "Kordlesspeakers" which receive audio signals via an infrared transmitter.
Bipolar speakers have been used in surround systems before, but arranged on the sides of the listening area pointing front and rear, or in front pointing front and rear.